The long-range goal of our research is to determine the nature of the regulation of protein breakdown so that it may be possible to intervene in a meaningful manner. Two models are currently being investigated in order to determine the factors and mechanisms involved in the control of degradation. The first model is liver regeneration in the partially hepatectomized rat. Currently the effects of hormone and amino acid levels on protein degradation, as measured by loss of label from liver protein following IP injection of H14CO3, are being investigated. The second model is the rapidly growing muscle protein of young chicks. Unlike rats, in which both synthetic and degradative rates appear to be correlated with the growth rate, the chick appears to have synthetic rates that are always rapid and somewhat independent of the growth rate, while the degradative rates appear to be lower in fast growing birds. Protein synthesis will be measured by a new technique developed here, that of feeding the labelled amino acid agar-gel diets. Degradation will be measured in both total and specific proteins in muscles by monitoring the loss of labelled amino acids from protein under conditions whereby the extent of amino acid recycling is minimized or measured. BIBLIOGRAPHIC REFERENCES: CHEE, Po Yok and Robert W. Swick. Effect of Dietary Protein and Tryptophan on the Turnover of Rat Liver Ornithine Aminotransferase. J. Biol. Chem. 251:1029-1034, 1976. Garlick, P.J., T.L. Burk and R.W. Swick. Protein synthesis and RNA in tissues of the pig. Am. J. Physiol. 230:1108-1112, 1976.